In addition to quality assurance of a product during working in manufacturing, measurement of the inner surface shape of a pipe body is required in order to shorten working hours. Heretofore, there is known an inner surface shape measuring device that applies a laser beam to an inner surface of an object to be measured, and measures the inner surface shape or the inner diameter of the object to be measured from the reflected light beam.
There is JP-A-2008-304407 (PATENT LITERATURE 1) as a background art of this technical field. This literature describes “the reflection surface of a prism is fixed on the axis of a laser beam emitted from a laser displacement gauge, so that operation such as adjustment of placement of the laser displacement gauge and the prism is not necessary, and an inner diameter D can be measured by simple operation, even in a case where an object to be measured is changed, or the shape of an inner peripheral surface to be measured is changed. Additionally, the reflection surface of the prism is disposed so as to protrude from a supporting body, and therefore even in a case where the diameter of the inner peripheral surface of the object to be measured is small, and the whole of a measuring unit cannot be guided in the inner peripheral surface, the inner diameter of the inner peripheral surface can be measured by guiding of the reflection surface of the prism”.
Additionally, there is JP-A-2011-196899 (PATENT LITERATURE 2). This literature describes “laser displacement sensors each including a light-emitting unit that emits a laser beam toward the inside of a pipe body in the axial direction of the pipe body, and a light-receiving unit that receives a reflected light beam returning from the inside of the pipe body are mounted at three places in the peripheral direction on the proximal end side of an arm whose distal end side is inserted in the pipe body, prisms, each of which changes, toward the inner diameter surface of the pipe body of the outside at a right angle, the direction of the laser beam emitted from each light-emitting unit, and changes, toward the proximal end side of the arm, the direction of a part of the reflected light beam on the inner diameter surface of the pipe body of the laser beam directed to the outside, are mounted in the peripheral directions identical to the peripheral directions of the respective laser displacement sensors on the distal end side of the arm, and an operating means for obtaining reflection positions of the laser beams on the inner diameter surface of the pipe body from detected values of the respective laser displacement sensors, and operating the inner diameter of the pipe body from the obtained three reflection positions is provided”.